1. FIELD OF THE INVENTION
The present invention relates to a circuit for driving a liquid crystal panel which uses a dot-matrix display method for displays of pocket televisions, lap-top computers or the like.
2. DESCRIPTION OF THE RELATED ART
FIG. 4 is a block diagram illustrating the construction of a part of a conventional circuit for driving a liquid crystal panel. In this figure, reference numeral 1 denotes a liquid crystal panel which uses a dot-matrix display method in which thin film transistors (TFTs) are used in switching pixels. An equivalent circuit of the switching matrix section thereof is shown in FIG. 5. Reference numerals 2 denote liquid crystal elements provided in each of the pixels on the matrix intersection points of the scanning electrodes (gate lines G.sub.1, G.sub.2, . . . ,G.sub.m) and signal electrodes (source lines S.sub.1, S.sub.2, . . . , S.sub.n). One end of each of the liquid crystal elements is connected to a common electrode terminal T.sub.c1. Reference numerals 3 denote TFTs, each of which is provided in each pixel and employed as a switching element for driving a corresponding liquid crystal element 2. The gates thereof are connected to gate lines G.sub.1, G.sub.2, . . . , G.sub.m for each line, and the sources thereof are connected to source lines S.sub.1, S.sub.2, . . . , S.sub.n for each row. Reference numerals 4 denote capacitors for storing signal charges for one vertical synchronization period, each of which capacitors is provided in each pixel. One end of each of the capacitors is connected to the drain of its corresponding TFT 3, and the other end is connected to a common electrode terminal T.sub.C2.
In FIG. 4, reference numeral 6 denotes an integrated circuit (IC) for driving the liquid crystal panel 1. In this driving IC 6, reference numeral 7 denotes a shift register for outputting a horizontal synchronization signal on the basis of which a signal electrode to which image data is input, is selected; reference numeral 8 denotes a sample hold circuit for sample-holding image data inputted on the basis of the horizontal synchronization signal outputted from the shift register 7; and reference numeral 9 denotes an amplifier for current-amplifying signals outputted in parallel form from the sample hold circuit 8. Signals outputted in parallel form from the amplifier 9 are input to the source lines S.sub.1, S.sub.2, . . . , S.sub.n.
With the construction described above, to display an image on the liquid crystal panel 1, gate lines G.sub.1, G.sub.2, . . . , G.sub.m on the liquid crystal panel 1 are scanned in sequence by a linear sequence method in order to simultaneously turn on all TFTs 3 on one gate line G. In synchronization with this scanning, a signal charge is supplied via source lines S.sub.1, S.sub.2, . . . , S.sub.n from the driving IC 6 to a capacitor 4 corresponding to a pixel to be displayed from among capacitors 4 connected to the drains of the turned-on TFTs 3. This signal charge continues to excite the liquid crystal element 2 of the corresponding pixel until the next scanning is performed. As a result of repeating the operations explained above, a desired image is displayed on the liquid crystal panel 1.
In a case where the liquid crystal panel 1 is used as a display of a pocket television or the like, a voltage for driving the liquid crystal panel 1 is an analog voltage since video signals are handled. However, when the liquid crystal panel 1 is driven by an analog voltage, there is a limitation on the scanning speed. Accordingly, as described above, the driving IC 6 must be formed of a number of functional elements, such as the shift register 7, the sample hold circuit 8, or the amplifier 9. As a result, as the liquid crystal panel 1 becomes larger, so does the driving IC 6. A drawback is that the surface for mounting parts becomes large, and the costs are increased.
To realize a high-resolution or large-screen display by using the liquid crystal panel 1, high response to high-frequency signals is required. However, because the analog amplifier 9 is used, there is the problem that a limitation is imposed on the liquid crystal panel 1 due to the operating frequency of the signals.
The present invention has been accomplished in light of the above-described circumstances. An object of the present invention is to provide a circuit for driving a liquid crystal panel in which even if the liquid crystal panel is formed into a large screen capable of realizing a high resolution display with a simple construction, there is no appreciable increase of either the mounting surface for the parts or the cost of production.
To this end, according to the present invention, there is provided a circuit for driving a liquid crystal panel in which a plurality of scanning electrodes and a plurality of signal electrodes which cross the plurality of scanning electrodes are provided, by sequentially scanning the plurality of scanning electrodes and supplying image data to the plurality of signal electrodes, the circuit comprising: a conversion section for converting image data to digital data binarized according to the gradation of the image data; a horizontal scanning signal generation section for generating horizontal scanning signals used to select a signal electrode to which the digital data is input; capacitor groups, each of which capacitor groups being formed of capacitors the number of which corresponds to the number of bits of the digital data; selection sections, provided in correspondence with the plurality of signal electrodes, for selecting none or at least one capacitor from among the capacitor group on the basis of the horizontal scanning signals and the digital data; a power-supply section for supplying an electric charge to each of the capacitor group; first switch sections, provided in correspondence with the plurality of signal electrodes, for charging selected capacitors only by the selection sections of the capacitor group in every horizontal synchronization period; and second switch sections, provided in correspondence with the plurality of signal electrodes, for connecting all capacitors of the capacitor group, including capacitors which are not selected by the selection sections, to corresponding signal electrodes and supplying the electric charge which has been charged to the signal electrodes, in every horizontal synchronization period.
With the above-described construction, to display an image on a liquid crystal panel, a plurality of scanning electrodes are scanned in sequence, and the operation set forth below is performed when image data is supplied to a plurality of signal electrodes.
First, image data is converted by the conversion section into digital data binarized according to the gradation of the image data. Next, a signal electrode to which the digital data is input, is selected on the basis of the horizontal scanning signals generated by the horizontal scanning signal generation section. Then, none or at least one capacitor is selected by the selection sections from among the capacitor groups on the basis of the horizontal scanning signal and the digital data. As a result, the first switch sections charge capacitors of the capacitor group only selected by the selection sections in every horizontal synchronization period. The second switch sections connect all capacitors of the capacitor groups, including capacitors which are not selected by the selection sections, to corresponding signal electrodes and supply the electric charge which has been charged to the signal electrodes, in every horizontal synchronization period. A desired image is displayed on the liquid crystal panel by repeating the above-described operations.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.